The present invention concerns a guide roll for a porous belt, such as a drying wire, specifically for machines used in the manufacture or treatment of a material web, such as a paper web. Guide rolls of this type may include a hollow stationary air guide box, about which rotates a perforated roll shell having at least two air chambers that extend across the length of the guide roll. One of the air chambers serves the discharge of blowing air, and the other serves the intake and removal of suction air.
Guide rolls of this general type are known and are described, for example, in U.S. Pat. No. 4,510,698 and TAPPI Journal September, 1987, P. 65-69, FIG. 10. In view of their function, these guide rolls are also called suction/blowing rolls.
The purpose of guide rolls of this type is to supply fresh air to the pockets of a two-screen drying section of a paper machine, and at the same time to evacuate the air saturated with water vapor. This serves to avoid transverse flows in the pockets, flutter of the web edges and disuniformity in the moisture cross profile and is shown in the TAPPI article at P. 67. These problems occur with greater frequency as the operating speed of the machine, such as the paper machine, is increased.
The blowing zone is preferably located in the approach area of the guide roll and the suction zone in the departure area. However, a reverse arrangement is possible as well. To enable the blowing air supplied to the porous belt, for instance to the drying wire of a paper machine, to absorb a maximum of water vapor, it is preheated. In comparison, the temperature of the suction air is lower.
In practice, blowing air and suction air thus have very different temperatures. This temperature difference may well amount to 30.degree.-40.degree. and more, when the overall system is started up again cold or after a web break. The temperature differences are also large and, as the case may be, unevenly distributed when the web, specifically the paper web, is not yet passing through the drying section, for instance during the start-up of the paper machine.
Due to the temperature differences, deformations and stresses can occur in the air guide box of the guide roll. This can result in friction losses and/or destruction of the seals which separate the blowing zone and the suction zone from each other.
The problem underlying the present invention consists in providing a guide roll of the categorical type wherein deformations of the air guide box due to the temperature between the blowing air and the suction air are extensively avoided, and where there are more possibilities than before regarding the selection of size, number and arrangement of the blowing and suction zones. It is therefore desired to provide a guide roll which allows a better adaptation to different operating conditions, such as temperature differences and/or different applications.